GH Actions: switch to Coveralls action runner to upload reports

Simplify the code coverage workflow by removing the dependency on the `php-coveralls/php-coveralls` package and switching to the `coverallsapp/github-action` action runner, which, as of the release of the [0.6.5 version of the Coverage Reporter](https://github.com/coverallsapp/coverage-reporter/releases/tag/v0.6.5) now natively supports the Clover format.

.github/workflows/test.yml

@@ -105,13 +105,9 @@ jobs:
       - name: Run the unit tests with code coverage
         run: composer coverage
 
-      # Global install is used to prevent a conflict with the local composer.lock.
-      - name: Install Coveralls
-        if: ${{ success() }}
-        run: composer global require php-coveralls/php-coveralls:"^2.6.0" --no-interaction
-
       - name: Upload coverage results to Coveralls
         if: ${{ success() }}
-        env:
-          COVERALLS_REPO_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        run: php-coveralls -v -x build/logs/clover.xml
+        uses: coverallsapp/github-action@v2
+        with:
+          format: clover
+          file: build/logs/clover.xml